Conventionally, in a color printer or the like, processing of separating image signals in RGB colors to color components of color material such as ink used in the color printer (hereinbelow referred to as “ink color separation processing”) is realized by a construction as shown in FIG. 30. In FIG. 30, reference numeral 1601 denotes a luminance/density converter which generates CMY signals as density signals from the RGB signals as luminance signals; numeral 1602 denotes a UCR/BG processor which performs under color removal processing based on the CMY signals inputted from the luminance/density converter 1601 to generate a black (K) signal and outputs C′, M′, Y′ and K′ signals; numeral 1603 denotes a BG amount (under color addition amount) setting unit; and numeral 1604 denotes a UCR amount setting unit.
Next, operations of the respective elements will be described. The luminance/density converter 1601 converts the input luminance information (8-bit data) R′, G′ and B′ into CMY signals based on the following expressions.C=−α log(R′/255)  (1)M=−α log(G′/255)  (2)Y=−α log(B′/255)  (3)(α: an arbitrary real number)
Next, the CMY data are converted based on a value β(Min(C,M,Y),μ) set in the BG amount setting unit 1603 and a UCR coefficient μ (%) set in the UCR amount setting unit 1604, toC′=C−(μ/100)×Min(C, M, Y)  (4)M′=M−(μ/100)×Min(C, M, Y)  (5)Y′=Y−(μ/100)×Min(C, M, Y)  (6)K′=β(Min(C, M, Y), μ)×(μ/100)×Min(C, M, Y)  (7)Note that Min(C,M,Y) indicates a minimum value of the CMY, and β(Min (C, M, Y), μ) is a real number which is changed based on Min(C, M, Y) and μ. The addition of black (K) ink is set by this value.
As the UCR amount and BG amount greatly influence the color reproduction range of the color printer and graininess of printer in correspondence with addition of black (K) ink i.e. under color, they are very important parameters for the color printer.
However, in the conventional art, the UCR amount is uniformly set based on the product of the UCR coefficient μ and Min(C,M,Y), and the ink amount of gray line always has a constant CMY ratio. Accordingly, the color tone of gray line differs depending on characteristics of respective ink and print medium.